Most of us think that the NWS and non-NWS data-
bases are of comparable value and both should be used
when developing ground snow loads.
The "all values" plot provides a good indication of
the "correct" answer in most cases, but in a few cases
it is not a very good indication. Thus, simply using the
"all values" answer is not recommended.
The three structural engineers involved chose to
somewhat modify the analytical procedure developed
by CRREL, each in his own way. Nonetheless, when
coupled with our anonymous feedback process, it was
easy for us to reach a consensus in almost all cases.
Stations with Pg/Pmax ratios greater than about 1.5
were given little weight and those with ratios above
about 1.7 were largely discounted in our analysis. We
determined that the log-normal distribution does a poor
job of predicting extreme values for such stations. Sta-
tions with Pg/Pmax ratios less than about 0.9 appear to
create similar problems.
An elevation correction factor of 2.1 lb/ft2 per 100
ft (0.33 kN/m2 per 100 m) works well for New Hamp-
shire to an elevation of about 2500 ft (about 762 m).
This factor may increase at higher elevations. It should
not be assumed to apply in other parts of the country.
Based on what we learned by conducting the 140
case studies in the CS zone, we think it is important to
do case studies for the 102 New Hampshire towns not
in that zone. We will begin that work in the near fu-
ture.
The case study process involves a more detailed
examination of an area than was achieved some years
ago when the national snow load map was made by
two of us. Thus, the case study process, in all likeli-
hood, produces a more accurate ground snow load.
In "CS" areas on the national map, case studies are
required. In other areas where mapped values have
elevation limits or change rapidly within short dis-
tances, case studies are recommended.
8 ACKNOWLEDGEMENTS
Financial support was provided by 14 New Hampshire
structural engineering companies, by the New Hamp-
shire Building Officials Association, and by the U.S.
Army Corps of Engineers. Over 75% of the work re-
ported here was done by volunteers.
REFERENCES
American Society of Civil Engineers, 1996. Minimum design
loads for buildings and other structures. ASCE Standard 7-
95. New York, NY.
Tobiasson, W. & Greatorex, A,. 1997. Database and methodol-
ogy for conducting site specific snow load case studies for
the United States in Snow Engineering, Recent Advances. In
Proceedings of the Third International Conference on Snow
Engineering), Sendai, Japan, 1996. Rotterdam: A.A. Balkema.
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